1 Accurate Kohn-Sham potential for the 1s2s 3S state of the helium atom: Tests of the locality and the ionization-potential theorems Sten Salomonson, Fredrik Moller¨ and Ingvar Lindgren Abstract: The local Kohn-Sham potential has been constructed for the 1s2s 3S state of the helium atom, using the procedure proposed by van Leeuwen and Baerends (Phys. Rev. A49, 2138 (1994)) and the many-body electron density, obtained from the pair-correlation program of Salomonson and Oster¨ (Phys. Rev. A 40, 5559 (1989)). The Kohn-Sham orbitals reproduce the many-body density very accurately, demonstrating the validity of the Kohn-Sham model and the locality theorem in this case. The ionization-potential theorem, stating that the Kohn-Sham energy eigenvalue of the outermost electron orbital agrees with the negative of the corresponding many-body ionization energy (including electronic relaxation), is verified to nine digits. A Kohn-Sham potential is also constructed to reproduce the Hartree-Fock density of the same state, and the Kohn-Sham 2s eigenvalue is then found to agree with the same accuracy with the corresponding Hartree-Fock eigenvalue. This is consistent with the fact that in this model the energy eigenvalue equals the negative of the ionization energy without relaxation due to Koopmans’ theorem. Related calculations have been performed previously, particularly for atomic and molecular ground states, but no one of matching accuracy. In particular, our results demonstrate that there is no conflict between the locality of the Kohn-Sham potential and the exclusion principle, as claimed by Nesbet (Phys. Rev. A58, R12 (1998)). PACS Nos.: 31.15Ew, 31.15Pf, 02.30Sa Resum´ e´ : French version of abstract (supplied by CJP) [Traduit par la redaction]´ Density-functional theory (DFT) is based upon the Hohenberg-Kohn (HK) energy functional [1, 2, 3] for an interacting system of electrons, moving in an external (nuclear) potential v(r), Z E[½] = FHK[½] + dr ½(r)v(r): (1) FHK[½] is the universal HK functional, which is in the constrained-search formulation [4, 5] ­ ¯ ¯ ® FHK[½] = minª!½ ª¯T^ + W^ ¯ª ; (2) where T^ is the kinetic-energy and W^ the electron-electron-interaction operators of the system. The wave function ª is assumed to be normalized and belonging to the Sobolev space H1(R3N ) [5, 6]. Were this functional known, minimizing it over normalized densities would yield the ground-state energy of the system, E0 = min½!N E[½] = E[½0]: (3) Sten Salomonson, Fredrik Moller¨ and Ingvar Lindgren. Department of Physics, Chalmers University of Technology and the Goteborg¨ University, Goteborg,¨ Sweden Can. J. Phys. : 1–5 () NRC Canada 2 Can. J. Phys. Vol. , It is easy to show that the variation principle works also for the lowest eigenstate of certain symme- try [7], and the principle has recently been extended also to more general excited states [8, 9]. According to the Kohn-Sham (KS) model [10], the electron density of the interacting system can be generated by a system of noninteracting electrons, moving in the local Kohn-Sham potential, vKS(r), £ ¤ 1 2 ¡ 2 r + vKS(r) Ái(r) = "i Ái(r): (4) The energy functional for this system is Z EKS[½] = TKS[½] + dr ½(r) vKS(r); (5) where ­ ¯ ¯ ® TKS[½] = min©!½ ©¯T^¯© ; (6) and © is a single Slater-determinantal wave function. The KS potential can be expressed by means of functional derivatives as ±F [½] ±T [½] v (r) = HK ¡ KS + v(r): (7) KS ±½(r) ±½(r) It has been proven by Englisch and Englisch [11, 12], based on works of Lieb [5], and recently recon- firmed by van Leeuwen [6], that the KS potential is under general conditions strictly local, although there are objections to that statement in the literature [13, 14, 15]. In some of our previous works we have also analyzed the differentiability of density functionals and demonstrated the locality under gen- eral conditions [16, 17, 18]. In the present work we shall, in addition, give strong numerical support of the locality theorem. More details of this work will be published elsewhere [19]. The orbitals of the KS model generate the exact density but were originally believed to have no other physical significance. It has been shown, though, that the negative of the eigenvalue of the out- ermost electron orbital (HOMO) equals the ionization energy of the system. This was first shown by Perdew et al. [20, 21] and is known as the ionization-potential theorem. This has been challenged by Kleinman [22], but there are numerous calculations supporting the theorem [23]. In our present work this theorem is verified with much higher numerical accuracy than in any other calculation known to us. Generally, the HK and KS energy functionals and the KS potential are not known, but more or less sophisticated approximations exist and are widely used in quantum-mechanical calculations [24, 25]. In certain cases, however, the electron density is accurately known, either from experiments or from ab initio calculations, and then the KS potential can be constructed with the corresponding accuracy. Essentially two schemes have been developed for this purpose, based upon works of Zhao and Parr [26, 27, 28] and of van Leeuwen and Baerends [29], respectively. In the present work we have constructed accurate KS potentials for the lowest states of the helium atom, using the scheme of van Leeuwen and Baerends. In this procedure, the electronic part of the Kohn-Sham potential, vel(r), defined by (in atomic units) Z v (r) = ¡ + v (r) + const:; (8) KS r el is obtained by iterations, using the formula i+1 ½i(r) i vel (r) = vel(r); (9) ½ex(r) where ½ex(r) is the exact many-body density and ½i(r) is the density generated with the potential i vel(r). NRC Canada Salomonson et al. 3 The electron density of the helium atom is in the present work generated by using the pair-correlation program of Salomonson and Oster¨ [30]. The wave function is here expanded in partial waves, and after convergence the wave function is for this two-electron system virtually exact. The convergence criteria in the iterative process (9) was set so that the iterated density should agree with the exact density within one part in 109 at all points. The rate of convergence is usually quite slow in this process — several thousands of iterations were often needed to achieve this kind of convergence. In order to improve the convergence rate and avoid ’oscillations’, it was sometimes helpful to take some average of the last two iterations as the input for the next one. It was also found important to keep the electronic part of the potential positive at all points during the iterations by adjusting the constant in the expression (8). After convergence the constant is adjusted so that the potential approaches zero at infinity. 1.2 1 0.8 0.6 0.4 0.2 RADIAL ELECTRON DENSITY 0 0 2 4 6 8 10 RADIUS r/a 0 Fig. 1. The Kohn-Sham density (dots) superimposed on the many-body density (solid line) for the 1s2s 3S state of the helium atom (atomic units). The final KS density for the 1s2s 3S system is shown in Fig. 1, superimposed on the exact many- body density. With the resolution of this figure, the two densities are indistinguishable. The corre- sponding KS potential is shown in Fig. 2. The potential has a ’bump’ near the node of the outermost electron, which is characteristic of ’optimized local potentials’ [31]. This is an effect of the electron self interaction (SIC), which varies strongly near the node of the valence wave function due to the ½1=3 dependence [32, 33]. The fact that we can generate the the exact many-body density in the Kohn-Sham model with a single local potential, proves the locality theorem in this case — at least within our numerical accuracy of nine digits. In order to test the ionization-potential theorem mentioned above, the eigenvalue of the Kohn- Sham 2s orbital is compared with the many-body ionization energy. According to this theorem these quantities should be equal, with opposite signs. We have found the KS eigenvalue to be -0.175 229 3794 H to be compared with our many-body ionization energy (including relaxation) of 0.175 229 3794 H, i.e., a numerical agreement to nine digits also in this case. These values agree also very well with the value 0.175 229 3782 H, obtained by Pekeris [34], using accurate Hylleraas-type wave functions (uncorrected for relativity, mass-polarization and QED effects). NRC Canada 4 Can. J. Phys. Vol. , 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 ELECTRONIC ENERGY POTENTIAL 0 0 2 4 6 8 10 RADIUS r/a 0 Fig. 2. The electronic part of the Kohn-Sham potential for the helium 1s2s 3S state. Note the ’bump’ near the node of the 2s orbital, characteristic of ’optimized’ local potentials [31] (atomic units). As a further test we have constructed the Kohn-Sham potential that reproduces the Hartree-Fock (HF) density of the same state. This density is easily generated by solving the standard HF equations and inserted into the generating formula (9) in place of the many-body density. The resulting KS potential was found to yield a 2s energy eigenvalue of -0.174 256 072 H, which agreed with the HF value to all digits given. This represents further tests of the locality and ionization-potential theorems. A comparison between the ionization energy and Hartree-Fock eigenvalue shows that the effect of relaxation enters here already in the third decimal place. This demonstrates that this effect is reproduced to six digits by the Kohn-Sham eigenvalue.